Revealing mistake: After Dante's Peak explodes for the final time, the pyroclastic cloud rolls down the mountain toward the town. Harry shouts "Don't look back, kids, don't look back." and then we are shown a wide shot of the truck zooming down the ash-choked streets. Unfortunately, it's painfully obvious that this is either a miniature or CGI shot, because the truck isn't leaving any tracks behind it or trailing any clouds of ash despite its high speed.
Plot hole: After the volcanic eruption, there's a scene where we see our heroes going across a lake, which has been turned to acid by the lava. If this acid is strong enough to corrode the propeller, then surely the fumes would be destroying their lungs, in which case they should be dead, or close to it.
Other mistake: The first time Rachel brings the team coffee, she has six coffees on a tray. There's five cappuccinos and one regular coffee for Harry. On the tray there is five coffees on one side and one coffee at the other side, obviously being Harry's regular coffee, so that Rachel would be able to tell the them apart. But when she gives Harry a coffee, she gives him one of the five standing together, so he obviously got one of the cappuccinos.
Plot hole: After the pyroclastic explosion Rachel, Harry and the kids get into the mine. There are a lot of things from Rachel's son in there: Sodas, chips, tapes and a tape player/recorder among them. There is a shot in which we can see a pile of tapes on the player, in almost perfect order. But if we consider that the whole town collapsed because of the earthquake occurred before the eruption, then those tapes must have fallen down, surely?
Factual error: In the scene where Paul and the National Guardsman are leaving the motel for the final time after packing up, Paul follows the two Humvee's out of town. Being a Guardsman myself, I know for a fact that all civilians would be in front and Humvees would be following them. It is standard operational procedure.
Continuity mistake: Near the start of the movie, during the violent eruption when the major characters have quickly loaded their gear into the truck and are trying to escape, the shot changes back and forth from looking into the cabin of the truck from the front and then looking forward out through the windscreen. The windscreen wipers are running at very different speeds. From inside, they run very fast (probably so you can see out through the mud) and very slowly from outside (probably to accentuate the mud etc).
Add timeNeil Glassford
Deliberate mistake: In the shot right after Harry successfully maneuvers out of the river, the truck he's driving should still have water pouring out of it. When he was initially pulling out, the water in the truck was still up to the bottom of the steering wheel, and we can see it draining out, but it would take much longer than a second or two for the water to fully drain out- the truck's completely dry and not leaking at all.
Factual error: The scene in which the main characters are in the ranger station truck and are driving across the lava flow, you see the tire getting absolutely covered in lava, flinging it inside the wheel well, plus the bottom of the truck is basically only an inch or two above it. How did the gas tank and the fuel lines survive that miraculously without melting or bursting? The heat would literally boil the fuel inside the lines and the truck would no longer be able to run.